The present invention is directed to methods and apparatus for using ultrasound with contrast agent. More particularly, the present invention is directed to methods and apparatus for using ultrasound to produce regions of a target area in which the concentrations of contrast agent are different.
Contrast agents may be used with diagnostic ultrasound to improve the image quality of a target area. Typically, contrast agents contain small particles, such as microbubbles, with high scattering capability. By injecting contrast agent into the bloodstream of a patient, contrast agent flows into the tissue to be imaged and ultrasound waves directed at the imaged tissue are scattered, thereby increasing the signal-to-noise ratio. The increased signal-to-noise ratio improves the quality of the ultrasound images, whether in the 2D, M, Doppler, or color Doppler modes.
Contrast agents have other benefits besides image enhancement. The temporal dynamics of some contrast agents provide information about blood circulation. The signal intensity, and thus the image brightness, is typically greater for greater contrast agent concentrations. After the initiation of contrast agent injection, the image brightness increases over time up to a saturation level. The rate of that change in image brightness is typically related to the rate of increase in contrast agent concentration.
Blood perfusion or local blood supply may be an important aspect of a patientxe2x80x9ds medical condition in a given body region. Blood perfusion has been conventionally estimated using contrast agent by measuring the time required for the image of a given body region to reach a brightness level associated with a pre-selected reference level.
Blood perfusion measurements may be used for specific clinical purposes. For detecting malignancies, the blood supply in malignant tissue is higher than in surrounding body areas. Thus, malignant tissue can be detected because the brightness of the image of malignant tissue increases faster and reaches the saturation level faster than healthy tissue, following injection of contrast agent. For detecting ischemic myocardial heart muscle segments, the pathological region is characterized by a slow rate of increase in image brightness following injection of contrast agent. This is so because myocardial segments suffer from a deficiency in blood supply.
Conventional methods, such as the two examples above, are based on the measurement of the time for the contrast agent concentration to rise. The concentration rise time is a relative parameter, so the diagnostic conclusion may be based on a comparison between defective tissue portions and healthy tissue portions.
In one method of measuring perfusion, the concentration rise time measurement should start from a low contrast agent concentration. One typical method of measuring rise time from a low contrast agent concentration is to start brightness measurements at the beginning of the contrast agent injection. Such a method is not very accurate because excessive time may be needed for contrast agent to be delivered to the target area by main blood flow. Also, the time for contrast agent to be delivered to the target area by main blood flow can vary for different body parts due to blood vessel structure. Delivery time should be taken into account in the calculation of perfusion rates when brightness is measured from the beginning of the contrast agent injection. Consequently, the accuracy of the perfusion rate calculations is decreased significantly when brightness measurements are started at the beginning of the contrast agent injection.
Another way to measure rise time is to use ultrasound to destroy contrast agent bubbles in a xe2x80x9cflash.xe2x80x9d A flash is a relatively powerful ultrasound scan comprising a frame or frames capable of destroying contrast agent. A flash may be characterized by a number of parameters including energy, frequency, or pulse duration. Even an ultrasound flash or burst of moderate amplitude is capable of destroying bubbles because of the low stability of bubbles in many contrast agents.
The starting point of a rise time measurement may be defined by the end of a flash. The flash produces a clean region, which is the target tissue while the target tissue has relatively little or no contrast agent. The clean region has a minimal brightness that corresponds to a suitable starting point for rise time measurement. Continuous contrast agent infusion maintains a high, stable level of contrast agent generally throughout the body except for the clean region just after the flash. After the flash, the contrast agent penetrates the clean region at the rate of the local blood perfusion, increasing the brightness of the clean region until the clean region is no longer clean but rather has reached a saturation point of contrast agent. Changes in the image brightness of the clean region may be monitored by using low power scanning or other methods of imaging to view that region from a period starting immediately after the flash.
The conventional flash method, however, has some deficiencies. Different parts of the target tissue may differ in brightness because of differences in depth. Time-varying acoustical shades might be due to contrast agent concentration variations in more shallow regions. These effects can disturb the estimation of rising time, because different portions of the target tissue reach the saturation level of brightness at different times.
Another deficiency of the conventional flash method results from the influence of image movement. Breathing, heart contractions, and probe movement are examples of types of image movements, and such movements can affect the local brightness in images of the target tissue. Heart movement in particular can be a major problem for cardiac imaging applications. Changes in local brightness thus might not only be attributable to variations in contrast agent concentration but also to complicated shaded-image movement.
The problem of shaded-image movement changing local brightness could be partially solved by ECG-triggered imaging. In ECG-triggered imaging, changes in brightness would be checked at only one time during a heart cycle.
Tissue tracking could be used to follow a tissue segment that is being analyzed, but this type of tissue tracking can be inaccurate, especially in the presence of time-varying contrast agent concentration. One reason for the inaccuracy is that the source of contrast agent for the tissue segment might not be local to the tissue segment, causing the measurements of the rate of change of brightness to reflect distance rather than actual perfusion rate.
Heart muscle health is often assessed by the observation of myocardium dynamics (i.e., contracting/stretching). One example is the Stress Echo procedure. The procedure is based on subjective estimation and depends strongly on a doctor""s -experience. The estimation is based on the image quality, which is very poor for some difficult patients. There is a need for an objective numerical criterion for contractility level.
In accordance with at least one preferred embodiment, a method is provided for producing zones with different contrast agent concentrations in a target. The method comprises the step of subjecting the target to an ultrasound flash capable of producing first and second target zones, the first target zone having a higher concentration of contrast agent than the second target zone. Another embodiment of the present invention is a method for producing an ultrasound image of a contrast agent infused target. The method comprises the steps of producing first and second target zones, the first target zones having a higher concentration of contrast agent than the second target zones, and forming an ultrasound image of the target, wherein the first and second target zones in the ultrasound image have different ultrasonic responses. The ultrasound flash may have a non-homogeneous energy distribution. The first and second target zones may form a plurality of stripes. Some embodiments of the present invention may be used to estimate blood perfusion rates. Some embodiments of the present invention may be used to estimate contractility of the heart.
Another aspect of the present invention is an ultrasound imaging system that includes a front-end controller (FEC), wherein the FEC controls a transducer to selectively radiate in different beam positions within a single frame at one of a plurality of energies. In the ultrasound system, a plurality of beam positions forming a first transmission region may have a first energy and a plurality of beam positions forming a second transmission region may have a second energy. Another embodiment of the invention is an FEC for use in a medical imaging system wherein the FEC controls a transducer to selectively radiate in different beam positions within a single frame at one of a plurality of energies. A further embodiment is an ultrasound system in which beam positions of a transducer are arranged to produce first and second transmission regions, wherein no beams are in the first transmission regions and at least one beam is in each of the second transmission regions.